Many flammable fluids and liquids commonly in use, for example in industry and agriculture, can be stored in and dispensed from relatively simple containers, such as drums of metal or other materials, provided the proper precautions are observed. Accordingly, the prior art teaches the use of safety valves threadedly associated with the drums, and capable of pressure-venting the drum to prevent explosions, and also providing vacuum relief by admitting ambient atmosphere to the drum, to equalize internal and external pressures and thereby permit the stored substance to be withdrawn through a spigot, or other separate passage, provided for that purpose.
Such valves have been found virtually to eliminate the danger of fire and explosion, by use of flame-retardant screens and by proper control of internal drum pressure. The valves also cut fuel loss, and have the additional important advantage that they operate automatically, and require no adjustment or replacement, even after years of use.
While very advantageous, such known vent valves are subject to a number of limitations and commercial disadvantages, which arise from the nature of their construction and the manner in which they are assembled. In this regard, the best vent valve devices hitherto available have been housed in a shell which is comprised of a number of castings which are inherently rather difficult to make, and therefore expensive. The best mode of associating the castings together, to provide the tight and secure shell which is required, has involved providing each with screw-threaded surfaces for disposition in threaded engagement with similar surfaces provided on mating castings. Provision of the several castings, and the formation of their threads is expensive, and assembly of the valve, including the castings, valve bodies, screens and springs, has proven to be a relatively complicated procedure.